Surface treatment of metal objects

ABSTRACT

A process for forming an outer diffusion surface layer on a metal substrate or member includes in a first activation stage of an inert particulate refractory material and a metal based material including metals and metal halides. An inert gas and hydrogen halide gas is introduced into the inert particulate refractory material and the metal based material to activate an outer surface of the metal based material. The metal substrate is pretreated to form a diffusion zone extending inwardly from the outer surface of the metal substrate having nitrogen forming an inner diffusion zone and an outer compound or white layer of an iron nitride, an iron carbide or an iron carbonitride compound without an oxide layer. A subsequent diffusion stage treats the metal substrate in an inert gas, in the absence of hydrogen halide gas to form the diffusion surface layer on the metal substrate.

The present invention relates to methods and apparatus for treating a metal substrate to achieve a diffusion surface layer on the substrate.

Metal surface treatments have traditionally comprised forming a nitrided surface on the substrate followed by a physical vapour deposition of a coating such as titanium, chromium nitride or carbon nitrocarburising onto the surface as an adhered coating. Some work has also been carried out where the surfacing material is diffused into the surface zone of the substrate simultaneously as nitrogen diffuses towards the surface making a chromium or titanium nitride or carbon nitride layer on the surface. The published patent specification of European Patent Nos. 0471276, 0252480, 0303191 and an International Publication Number WO/47794 disclose such treatment methods. Such methods are capable of providing a better performing surface treatment because, the surface layer is a diffusion layer and not simply a coating layer adhered to the substrate, however, practical control of the required materials and parameters to achieve this desirable result has proven to be quite difficult. The use of a halide gas such as HCl mixed with a reactive gas or a combustible gas such as hydrogen and/or ammonia leads to problems in the construction of the mixing gas panel. Further HCl and other halide gases are relatively expensive and extensive use of such gases can provide relatively expensive processing of a desired product. Also the halide gas can react instantly at low temperatures with ammonia forming solid ammonium chloride which may block the gas pipes and even leak back into the solenoid valves and flow meters of the gas delivery equipment causing blockages and potential damage to the equipment.

International patent application no. PCT/AU2006/001031 discloses treatment methods and treatment apparatus enabling a desired diffusion layer to be formed on a metal substrate product, the methods disclosed supply halide gas throughout a lengthy period of the processing and while the methods work satisfactorily, the processing cost is quite expensive due to the required volume of halide gas utilized.

The objective therefore of the present invention is to provide a method of forming a diffusion surface layer on a metal substrate in a more economical manner than with prior art processes while still retaining a reliable and safe processing of the metal substrate.

Accordingly, the present invention provides in a first aspect, a method of forming a diffusion surface layer extending inwardly of an outer surface of a metal substrate, said method including:

-   (i) in an activation stage, providing an activation treatment     furnace containing an inert particulate refractory material and a     metal based material for forming said diffusion surface layer, said     activation treatment furnace having a flow of an inert gas     introduced into the inert particulate refractory material and the     metal based material in the activation treatment furnace for a first     period of time to treat an outer surface region of said metal based     material in the presence of a hydrogen halide gas to form an     activated metal based material with an activated surface region; and -   (ii) in a diffusion stage, providing a diffusion treatment furnace     and introducing said metal substrate into said diffusion treatment     furnace, said metal substrate having been pretreated to form a     diffusion zone extending inwardly from the outer surface of the     metal substrate in which nitrogen has been diffused to form an inner     diffusion zone and an outer compound or white layer formed, at least     in part, by an iron nitride, an iron carbide or an iron carbonitride     compound without an oxide layer on said outer surface of the metal     substrate, treating the metal substrate in the diffusion treatment     furnace, sealed against the ingress of atmospheric air and under an     inert gas atmosphere, in the absence of hydrogen halide gas for at     least a second period of time, in the presence of said activated     metal based material, to form said diffusion surface layer on said     metal substrate.

Conveniently, the aforesaid method may further include:

-   (i) in a pretreatment stage, forming said diffusion zone extending     inwardly from the surface of the metal substrate in which nitrogen     has been diffused to form an inner diffusion zone and an outer     compound or white layer of an iron nitride, iron carbide or     carbonitride compound; and -   (ii) treating the metal substrate formed in said pretreatment stage     to either prevent formation of a surface oxide on said surface or to     remove any said surface oxide formed on said surface prior to said     metal diffusion stage.

In a preferred construction the aforesaid method may be carried out in a single treatment furnace where the diffusion treatment furnace also acts as the activation treatment furnace. The method can however, be carried out in different furnaces acting as the activation treatment furnace and the diffusion treatment furnace.

Preferably, the inert gas flow in the activation stage may be nitrogen and/or argon. Conveniently, the inert particulate refractory material utilized in the treatment furnace or furnaces might be aluminium oxide or silicon carbide.

Conveniently, when the diffusion treatment furnace contains an inert refractory particulate material, it is fluidized by a flow of an inert gas during the metal diffusion stage. Alternatively, such an inert refractory particulate material might be fluidized or at least partly fluidized by vibration means. Preferably, ammonia is not supplied to the diffusion treatment furnace during the metal diffusion stage.

In a particularly preferred embodiment, the second period of time is greater than the first period of time. In this manner, the relatively expensive hydrogen halide gas is used for much shorter periods to achieve the desired diffusion layer on the metal substrate. During the diffusion stage, the hydrogen halide gas might not be utilized at all but small amounts of the hydrogen halide gas could be used for short periods of time to reactivate the metal based material, if required. Typically, if required, the hydrogen halide gas might be pulsed for periods of no hydrogen halide gas provided in the retort and at least one period of hydrogen halide gas provided during the diffusion stage.

Conveniently an inert gas flow may be provided to the diffusion treatment furnace during the second period of time, the inert gas flow being variable from a zero flow rate to a flow rate at or above a minimum fluidization velocity for the diffusion treatment furnace.

Conveniently the operating temperature for the first and second periods for the treatment furnace or furnaces during the activation stage and the diffusion stage is between 500 and 750° C.

Preferably, in one embodiment the hydrogen halide gas flow may be supplied continuously to the activation treatment furnace during the first period of time. In a possible alternative, the hydrogen halide gas might be pulsed with periods of supply and periods of non supply during the first period of time. Conveniently, the hydrogen halide gas used might be selected from hydrogen chloride gas, hydrogen bromide gas, hydrogen fluoride gas or hydrogen iodide gas. The hydrogen halide gas when supplied to the activation treatment furnace or the diffusion treatment furnace is preferably mixed with an inert carrier gas (e.g. nitrogen and/or argon gas) externally of the treatment furnace or furnaces. Conveniently, when supplied, the hydrogen halide gas and the inert carrier gas enter the treatment furnace or furnaces at a lower region thereof.

In a further embodiment, the hydrogen gas might be created in the treatment furnace or furnaces by supply of ammonium chloride (NH₄Cl). Ammonium chloride might be supplied in solid or pellet form through a delivery tube or pipe whereby it is heated in the delivery pipe or tube to disassociate into nitrogen gas and hydrogen chloride (HCl) gas. An inert gas such as nitrogen or argon could also be supplied via the delivery tube such that the HCl gas is at least partly mixed with the inert gas by the time it enters the furnace. Such a delivery system might be used in either the activation stage or, if required, in the diffusion stage. If this delivery system is used, the operating temperature of the furnace might be close to 700° C. or even higher.

The metal based material for forming the diffusion surface layer may be chosen from at least one of:

-   (i) a solid metal or metal alloy; -   (ii) a metal or metal alloy coated on a substrate carrier; -   (iii) a particulate or powder metal or metal alloy; -   (iv) a metal or metal alloy coated on an inert particulate     refractory material; -   (v) a metal halide particle or powder (anhydrous or hydrated); and -   (vi) a metal halide material (anhydrous or hydrated) coated on an     inert refractory particulate material or a substrate carrier.     The metal based material might be selected from chromium, titanium,     vanadium, niobium, tantalum, tungsten, molybdenum, manganese, and     alloys thereof including ferrous based alloys, or metal halides     comprised of a metallic element of the aforesaid metals and a halide     selected from chlorine, bromine, iodine and fluorine.

The metal substrate is conveniently a ferrous based metal or a ferrous based metal alloy.

Conveniently nitrogen as an inert gas is introduced into the diffusion treatment furnace during the second period of time.

The term “metal substrate” is intended to refer to any metal part suitable for heat treatment made from ferrous based metal or ferrous based metal alloys.

In accordance with the method of this invention where hydrogen chloride is the halide gas used and chromium metal particles are used to form the diffusion surface layer, it is believed that hydrogen chloride causes an active chromium chloride layer on the surface of the aluminium oxide (inert fluidizing media) as well as on the chromium metal particles in the fluidizable bed furnace during the activation stage. During the metal diffusion stage of the process, a solid-state interaction between the activated chromium chloride and a nitrogen-rich ferrous surface of the metal substrate occurs to form the diffusion surface layer on the substrate. This occurs when the treatment furnace, typically a fluidizable bed furnace is substantially not fluidized by a flow of inert gas and also when the bed is fluidized. Fluidization of the bed can occur either by a suitable gas flow or by some vibration means as is known in the art. The process has considerable economic advantages as the hydrogen halide gas, typically hydrogen chloride is expensive and minimizing its use provides a much more economical process.

It is generally desirable that the outer portion of the diffusion zone (the white layer), be substantially free from porosity. The white layer will normally be an iron nitride, iron carbide and/or an iron carbonitride, typically either epsilon and/or the gamma form.

A preferred embodiment of the process of this invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional schematic view of a fluidizable bed furnace capable of use in the performance of the present invention;

FIGS. 2 and 3 are detailed cross-sectional views of seal arrangements capable of use with the fluidizable bed furnace shown in FIG. 1;

FIG. 4 is a graph showing Nitrogen (N), Chromium (Cr) and Iron (Fe) wt % concentrations against depth in the treated metal sample of Example 1 produced according to the present invention;

FIG. 5 is a graph showing Nitrogen (N), Chromium (Cr), Iron (Fe) and Copper (Cu) against depth in the activated chromium coated copper carrier substrate of Example 1;

FIG. 6 is a graph showing Nitrogen (N), Chromium (Cr) and Iron (Fe) wt % concentrations against depth in the treated metal sample of Example 1 not produced according to the present invention;

FIG. 7 is a graph showing Nitrogen (N), Chromium (Cr), Iron (Fe) and Copper (Cu) wt % concentrations against depth in the non activated chromium coated copper carrier substrate of Example 1;

FIG. 8 is a graph showing Chromium (Cr), Iron (Fe) and Nitrogen (N) wt % concentrations against depth in the treated metal sample of Example 2 produced according to the present invention;

FIG. 9 is a graph showing Chromium (Cr), Iron (Fe) and Nitrogen (N) wt % concentrations against depth in the treated activated Chromium sample utilized in Example 2 resulting from carrying out the process of this invention;

FIG. 10 is a graph showing Iron (Fe), Nitrogen (N) and Chromium (Cr) wt % concentrations against depth in the metal sample when not treated with a preactivated chromium sample as described in Example 2;

FIG. 11 is a quantitative depth profile showing Iron (Fe), Chromium (Cr), Nitrogen (N), Carbon (C) and Oxygen (O) wt % concentrations against depth in the metal sample treated according to the present invention utilising activated chromium powder as described in Example 3;

FIG. 12 shows the microstructure of the treated metal sample represented in FIG. 11 (Example 3);

FIG. 13 is an x-ray diffraction analysis showing the diffusion layer in the treated metal sample (Example 3) was predominantly CrN;

FIGS. 14 and 15 are a quantitative depth profile showing Chromium (Cr), Iron (Fe), Nitrogen (N), Carbon (C) and Oxygen (O) against depth in the respective treated metal sample as described in Example 4; and

FIG. 16 shows the microstructure of the treated metal samples as described in Example 4.

Reference will now be made to the drawings which schematically illustrate relevant parts of a fluidized bed treatment apparatus according to a preferred form of this invention, it being understood from the preceding disclosure that at least the pre-treatment stage of the heat treatment process need not be completed in fluidized bed heat treatment equipment and any other known heat treatment equipment could be used in this stage. Moreover, although it is desirable that the activation stage and the diffusion stage be carried out in the same fluidized bed heat treatment furnace, it is equally possible for separate fluidized bed heat treatment furnaces to be used for the activation and diffusion stages.

As illustrated in FIG. 1, the apparatus comprises a fluidized bed furnace 10 having an inner retort 11 containing a particulate inert refractory material 12 such as aluminium oxide (Al₂O₃), however, other such inert refractory materials can be employed. The furnace includes an outer insulating layer 13 and a heating zone 14 that might be heated in any conventional manner by combusting a fuel gas, by electrical resistance heating or by any other suitable means. In the drawings, the heating zone 14 is heated by a fuel gas supplied burner 16. At the bottom of the retort 11, a primary inert gas supply line 17 is provided for fluidizing the refractory material 12 when required. The gas supply line 17 leads to a gas distribution system comprised of a primary distributor 18 and a secondary distributor 19 typically of a porous material construction that is aimed at preventing streaming of the gas flow within the retort and thereby even fluidization and heat treatment. A further gas delivery line 20 is provided so that a halide gas and an inert carrier gas can be introduced into the bottom of the retort via a further distributor 21 separate from the distributors 18/19. A carrier inert gas line (e.g. nitrogen and/or argon) might be supplied via a line 70 with a hydrogen halide gas supplied via line 71 and mixed in a valve 72 before being delivered via line 20. The amount of inert gas delivered via lines 70 and 17 and the amount of hydrogen halide gas delivered via line 71 may be metered such that the gas quantity delivered to the furnace 10 is known. The distributor 21 might be positioned in the coarse refractory material zone 80 in the lower region of the retort 11. As an alternative, the delivery line 20 may enter through the bottom of the retort as shown in broken outline or elsewhere subject to the distributor 21 being located in the lower region of the retort. In this arrangement the delivery line 20 might pass upwardly as shown at 20′ and include one or more heating coils 81 before returning the halide and inert carrier gas to the distributor 21 in the lower region of the retort 11. The heating coil(s) 81 are conveniently just above or just within the coarse refractory material zone 80. It is preferred that the halide gas and the inert carrier gas be thoroughly mixed externally of the retort 11 and further that it be heated before the mixed gases enter the retort. Conveniently heating occurs by heat exchange with a region of the fluidized bed treatment furnace. With the illustrated arrangement in full line, heating of the externally mixed gases occurs as the line 20 passes downwardly through the heated refractory material in the retort. Other arrangements are equally possible. For example one or more coils of the delivery pipe might be provided in the line 20 within the retort. Alternatively, the delivery line 20 might pass through the heating zone 14 with one or more coils located in the zone 14. In yet another possible arrangement the premixed inert carrier gas and hydrogen halide gas might enter the furnace directly to be discharged via the distributor 21 without being preheated. Metering and mixing equipment (not illustrated in detail) is used to ensure proper proportions of halide gas and inert carrier/fluidizing gases are used in the activation stage of the treatment process.

An exhaust passage 22 leads from an upper region of the retort 11 whereby exhaust gases can escape in a controlled manner and be treated downstream (not shown) for safety purposes. It is possible for some of the refractory material to escape along this path and this material is conveniently collected in a grit collection box or container 23. From time to time it is possible for certain reaction products to solidify in this passage 22 which might lead ultimately to the passage becoming blocked. A scraper mechanism 24 may be provided to scrape such materials, preferably back into the collection box 23. Other approaches could be utilized rather than the illustrated physical scraper. For example, pulsed bursts of inert gas might be used from time to time to break up or move material in the exhaust passage 22 back into the retort 11. Conveniently particulate metal or metal alloy (when used in a treatment process) can also be introduced via the exhaust passage 22. A storage zone 25 for such particulate metal is provided with a metering valve or the like 26 to deliver a desired quantity of metal powder or metal coated particulate material into the passage 22. The scraper mechanism 24 if used or some pusher device might then be used to push this metal into the retort 11 when required. This is preferably done when the bed is slumped (i.e. not in operation) such that there is no or minimal gas flow in an outward direction along the passage 22.

As shown in FIG. 1, a first seal means 27 associated with a cover member 29 is provided around the upper access opening 28 leading to the inner zones of the retort 11. The first seal means 27 enables the retort 11 to be sealed against the ingress of atmospheric air during a treatment process. Features of the first seal means 27 are better seen in FIG. 2 or 3 where they are shown operationally with the cover member 29 for the upper access opening 28. The first seal means 27 comprises a first outer seal part 30 formed by a circumferential flange 31 on the cover member 29 engaging with a seal material 32 positioned between two circumferential and radially spaced flanges 33, 34 on a member 35 secured to the retort 11 and surrounding the access opening 28. The first seal means 27 further includes a second inner seal part 36 formed by circumferential flange 37 supported on the member 35 and engaging with a seal material 38 positioned between the outer flange 31 on the cover member 29 and a more inwardly located circumferential flange 39 carried by the cover member 29. The seal materials 32 or 38 may be any compressible seal material capable of operation at the relevant operating temperatures for the furnace, but may include ceramic fibre or VITON (registered trade mark) rubber material. When the first seal means 27 is operationally engaged as illustrated in FIG. 2 a, a seal zone 40 is established between the flanges 31 and 37. A gas distributor tube 41 is located in this zone 40 and is fed externally via a line schematically shown at 42 to deliver nitrogen, argon or some other inert gas to the zone 40 at a pressure whereby such gas will leak towards the retort opening 28 if leakage is possible thereby preventing ingress of atmospheric oxygen into the retort 11. The seal means 27 further includes a third seal part 43 formed by the inner circumferential flange 39 being engaged in a zone 44 containing inert refractory particulate material 45 typically of the same type as contained within the retort 11. The particulate material 45 may be fluidized by an inert gas supply delivered via line 46 to a distributor 47 therefor to assist at least entry of the flange 39 into the particulate material 45 as the cover member 29 moves to the illustrated closed position. To enable access to the retort 11, the cover member 29 is removed. This would occur, for example, when a treatment member (e.g. metal substrate) is introduced or withdrawn from the retort.

In the seal arrangement shown in FIG. 3, two annular flanges 82, 83 are provided upstanding from the peripheral retort part or member 35 defining a seal zone 84 therebetween. The flanges 82, 83 are welded or otherwise secured to the retort part 35 and are of differing heights to achieve the seal zone 84. The upper edges 85, 86 of the flanges 82 press into and seal with a suitable seal material 87 within an annular recess 88 in the cover member or lid 29. Preferably the upper edge 85 of flange 82 is marginally lower than the upper edge 86 of flange 83 whereby if gas leakage from the seal zone 84 occurs it will preferentially leak towards the inside of the retort 11 rather than externally of same. The seal material 87 might be the same type of material discussed above for seal material 32, 38 of FIG. 2 a. An inert gas delivery tube 42 is provided to deliver inert gas (eg nitrogen) to a distributor ring 41 within the seal zone 84 such that when the furnace 10 is in use and the cover member 29 is closed, the seal zone 84 is pressurized with an inert gas at a pressure higher than atmosphere and higher than within the retort. Gas leakage from the seal zone 84 “may” occur in both directions past the upper flange edges 85, 86 but preferentially, if leakage does occur at all, it will occur past the edge 85 back towards the retort. Thus the required atmosphere is maintained within the retort without permitting unwanted oxygen to enter same from the external atmosphere. Inwardly of the seal zone 84 a further annular flange 89 is provided with a heat insulating material 87 therebetween which can be the same material as the seal material 87 discussed above. Refractory particle material 91 can build up as shown in FIG. 3, but at a point where the slope of this material is about 60° to the horizontal, further such material will fall by gravity back into the retort 11, helped by any inert gas leakage inwardly past the flange edge 85. Thus escaping of refractory material from the retort is prevented or kept to a very low level. Conveniently the volume of the seal zone 84 is kept to a minimum to minimize inert gas usage. The lid or cover member 29 carries a treatment basket (or similar) support device 92 and the cover member 29 is conveniently at least insulated against heat loss. In some applications, particularly when batch processing, it may also be desirable to include cooling coils or tubes in the lid or cover member 29 to cool down the furnace 10 when desired at the end of a treatment operation. The lid or cover member 29 might also carry optionally, a plug 93 to minimize space above the treatment bed.

The process of this invention according to a number of preferred aspects will now be described. In a pre-treatment stage, a metal part (or substrate) to be treated is, subjected to a surface treatment known generally as nitriding or nitrocarburising. This can be achieved in a variety of different apparatus including salt baths, gas heat treatment apparatus, vacuum plasma equipment and fluidized bed furnaces. It is, however, desirable that the so-called white layer established via this first stage is substantially without significant porosity. Other desirable factors also relate to the concentration, depth and microstructure of the white layer including the lack of porosity therein.

When producing a nitrided or nitro carburised structure, two zones are produced. The first inner zone is the diffusion zone where nitrogen diffuses into the substrate through the diffusion zone from the substrate surface and increases the hardness of the substrate, and the second outer zone is the white layer which can consist of either the epsilon and/or the gamma layer as illustrated, for example, in international patent application no. PCT/AU2006/001031.

When the pretreatment stage of this process is carried out in a fluidized bed heat treatment furnace, control of same requires the supply to the bed of ammonia/nitrogen (for nitriding) and a carbon bearing gas (e.g. natural gas and/or carbon dioxide) for nitrocarburising. During nitrocarburising, it is important that some oxygen is involved in the process which may be contributed by a hydrocarbon gas, carbon dioxide and/or oxygen. Once this pre-treatment stage is completed satisfactorily, the part or substrate to be processed needs to be treated to ensure a surface oxide does not exist on the surface into which a metal is to be diffused. To obtain (or maintain) a suitable surface finish, one of the following options may be followed:

-   (i) the surface of the part or substrate might be mechanically     treated such as by repolishing and then kept under an inert     atmosphere before proceeding with the second stage; -   (ii) the surface of the part or substrate could be maintained fully     under an inert atmosphere or within a vacuum between the     pretreatment stage up to and including the activation and metal     diffusion stages; -   (iii) any surface oxide formed on the surface of the part or     substrate could be removed in the activation stage with a     combination of halide gas and hydrogen; or -   (iv) the surface of the part may be subjected to a wet abrasion     process where grit and air and water pressure can be varied to blast     the surface. This process selectively removes any cover layer while     retaining the desirable white layer.

In the activation stage of the process, the metal or metal based material to be surface diffused may be placed into and held in a fluidized bed furnace operated at a temperature below 750° C. and preferably no higher than 700° C. Conveniently the temperature is in the range of 500° to 700° C., typically about 575° C. The bed itself may include an inert refractory particulate material such as Al₂O₃ with the desired metal to be diffused into the surface in particulate or powder form in the bed or alternatively coating the inert refractory particles. Such metal should preferably comprise between 5 to 30 weight percent of the bed materials, i.e. the balance being the inert refractory material. The bed is then fluidized by a flow of halide gas (e.g. hydrogen chloride) and inert gas for a first period of time without the metal substrate to be treated. The inert gas may be argon and/or nitrogen in the presence of a separately introduced halide gas (e.g. HCl) premixed into an inert carrier gas stream (e.g. nitrogen and/or argon).

Preferably, the metal powders introduced into the bed should be of high purity and conveniently without a surface oxide. Thus measures need to be taken to prevent air contact before the powders enter the bed and while they remain in the bed itself. The gases used also need to be of high purity. Common inert gases capable of use in the process are high purity nitrogen (less than 10 ppm oxygen), high purity argon (less than 5 ppm oxygen), and for the first pretreatment stage processing, technical grade ammonia which has no more than 500 ppm water vapour and is further dried, for example by passing same through a desiccant before use. The hydrogen halide gas used may typically be a technical grade HCl although other hydrogen halide gases might be used.

The hydrogen halide gas typically will constitute between 0.2 and 3 percent of the total gas flow to the fluidized heat treatment bed furnace. The hydrogen halide gas flow needs to be closely regulated and mixed thoroughly with the inert carrier gas before it enters the bed. This is important to avoid non uniformity within the bed. The hydrogen halide gas may be preheated before it enters the bed to ensure that it is in its most reactive stage when it enters the bed. Preheating of the halide gas and the inert carrier gas has the benefit of enabling a further reduction in the amount of hydrogen halide gas required. The first period might typically be between 45 and 120 minutes, preferably between 60 and 90 minutes to produce an active layer on the diffusion metal and on the inert fluidizing media (aluminium oxide) in the bed. When chromium is used and the hydrogen halide gas is hydrogen chloride, the active layer will be chromium chloride.

At the end of this initial activating period, the pretreated metal substrate (pretreated as described above) is immediately introduced into the furnace bed or a furnace bed containing the activated metal based material and the flow of halide gas is then stopped. During this subsequent metal diffusion stage, the metal substrate on which the diffusion layer is to be formed is then held within the preactivated bed for a second period (typically 1 to 8 hours and preferably 4 to 8 hours) under an inert gas atmosphere. The bed is conveniently held at a temperature below 750° C. and conveniently in the range of 500° C. to 700° C., typically about 575° C. The fluidized bed in the metal diffusion stage may have minimal inert gas flow such that it is substantially slumped up to a high inert gas flow such that it is highly fluidized. The inert gas might be nitrogen. In some cases it might be desirable to include a pulsed halide gas flow during the second stage, if it is deemed the bed needs some reactivation.

It is generally desirable during treatment processes to maintain relatively uniform temperature levels in the bed, i.e. between the various heights in the bed. This may be achieved by including temperature monitoring means and varying the flow of the inert gases to the bed in response to sensed temperatures.

The metal or metal based material used to provide a metal to be diffused into the diffusion surface layer of the metal substrate to be treated may be chosen from at least one of a solid metal or metal alloy either in particulate form or one or more solid block members, a metal or metal alloy coated on a substrate carrier where the substrate carrier is in particulate form or as one or more solid block members where the substrate carrier will not, within the treatment conditions, react with the coating metal or metal alloy or the metal substrate being treated, a metal halide particle or powder (anhydrous or hydrated), and a metal halide material (anhydrous or hydrated) coated on a substrate carrier where the substrate carrier is in particulate form or as one or more solid block members where the substrate carrier will not, within the treatment conditions, react with the coating material or the metal substrate being treated. Conveniently the metal of the metal based material used to provide a metal to be diffused can be selected from chromium, titanium, vanadium, niobium, tantalum, tungsten, molybdenum, manganese, and alloys thereof including ferrous based alloys. Conveniently, the above referred to metal halides may be comprised of a selected metal as set out above and a halide selected from chlorine, bromine, iodine or fluorine. For example, CrCl₂ and CrCl₃ are soluble in water and ethanol to form a slurry whereby it could be painted on a suitable carrier substrate or the carrier substrate could be dipped into the slurry to form a suitable coating.

Several examples of preferred embodiments of the process of this invention will be described in the following.

EXAMPLE 1

A specimen of hardened and tempered (1020° C. autenitised and air cooled, double tempered at 575° C.) AISI H13 hot work tool steel with a diameter of 38 mm and thickness of 5 mm was nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of this specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of this nitrocarburised sample was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.

A 38 mm diameter 5 mm thick piece of pure copper was polished to a 1200 grade SiC finish prior to electrolytic hard chromium plating from a commercial supplier. A 2 micron pure chromium layer was produced by this method. Copper was chosen as a substrate carrier as Cr and Cu are essentially insoluble, and therefore the chromium layer will not decompose by diffusion into the copper specimen during heating. This chromium-plated sample was then immersed in a fluid bed heat treatment reactor of diameter 90 mm and depth 250 mm containing 3 kg of 99.99% purity alumina oxide powder of average particle size 125 microns. This fluid bed was heated to 575° C. under nitrogen and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% of the total gas flow. This “activation” stage continued for a duration of 1 hour. After this activation stage the chromium plated copper sample was cooled to room temperature in a flow of nitrogen.

Immediately after removal from the fluid bed reactor into ambient air conditions the hydrogen chloride activated chromium plated copper sample was physically coupled to the nitrocarburised sample, and a clamping pressure applied. This coupling was then placed in a fluid bed furnace and heated to 575° C. under nitrogen flow, held at this temperature for 4 hours, then cooled to room temperature under a nitrogen flow. This experiment was repeated, where the coupling consisted of chromium-plated copper without hydrogen chloride treatment as aforesaid and a nitrocarburised specimen. Upon uncoupling the two contacting surfaces were analysed for chemical composition using Glow Discharge Optical Emission Spectroscopy (GDOES).

It was found that by activating the surface of the chromium plated copper sample by use if hydrogen chloride gas, this surface reacted with the nitrocarburised specimen. Chromium transferred from the activated chromium-plated sample to the nitrocarburised specimen (FIG. 4), depleting chromium on the chromium-plated copper specimen (FIG. 5). In response to the enrichment of chromium on the nitrocarburised surface, nitrogen diffused to the surface to create a peak coinciding with the chromium peak (FIG. 4). Iron transferred from the nitrocarburised sample to the chromium-plated sample (FIG. 5). Correspondingly, the iron concentration was depleted on the nitrocarburised specimen (FIG. 4). In contrast, no reaction occurred between the non activated chromium plated copper and the nitrocarburised surface. No chromium enrichment of the nitrocarburised surface was observed (FIG. 6) or depletion of chromium from the chromium-plated copper specimen (FIG. 7). This example indicates the importance of hydrogen chloride surface activation of chromium to the transfer of chromium metal from a chromium source to a nitrogen-rich surface zone.

EXAMPLE 2

A specimen of hardened and tempered (1020° C. autenitised and air cooled, double tempered at 575° C.) AISI H13 hot work tool steel with a diameter of 38 mm and thickness of 5 mm was nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of this specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of this nitrocarburised sample was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.

A 38 mm diameter 5 mm thick piece of 99.99% purity chromium was polished to a 1200 grade SiC was immersed in a fluid bed reactor of diameter 90 mm and depth 250 mm containing 3 kg of 99.99% purity alumina powder of average particle size 125 microns. This fluid bed was heated to 575° C. under nitrogen and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% flow. This “activation” stage continued for duration of 1 hour. After this activation stage the chromium sample was cooled to room temperature in a flow of nitrogen.

Immediately after removal from the fluid bed reactor in to ambient air conditions the hydrogen chloride activated chromium sample was physically coupled to the nitrocarburised sample and a clamping pressure applied. This coupling was then placed in a fluid bed furnace and heated to 575° C. under nitrogen flow, held at this temperature for 4 hours, then cooled to room temperature under a nitrogen flow. This experiment was repeated, where the coupling consisted of chromium without hydrogen chloride treatment and a nitrocarburised specimen. Upon uncoupling the two contacting surfaces were analysed for chemical composition using Glow Discharge Optical Emission Spectroscopy (GDOES).

As per Example 1, by activating the surface of chromium by use if hydrogen chloride gas, this surface reacted with the nitrocarburised specimen. Chromium transferred from the activated chromium sample to the nitrocarburised specimen and nitrogen diffused to the surface to create a peak coinciding with the chromium peak (FIG. 8). Iron transferred from the nitrocarburised sample to the chromium-plated sample (FIG. 9). Correspondingly, the iron concentration was depleted on the nitrocarburised specimen (FIG. 8). In contrast, no reaction occurred between the chromium without prior activation and the nitrocarburised surface. No chromium enrichment of the nitrocarburised surface was observed (FIG. 10)

EXAMPLE 3

Two specimens of hardened and tempered (1020° C. autenitised and air cooled, double tempered at 575° C.) AISI H13 hot work tool steel with a diameter of 38 mm and thickness of 5 mm were nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of each specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of the nitrocarburised samples was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.

In a fluid bed reactor of diameter 90 mm and depth 250 mm 380 g of 99.99% purity chromium powder of average particle size 80 microns was mixed with 3.4 kg of 99.99% purity alumina powder of average particle size 125 microns.

This fluid bed was heated to 575° C. under high purity nitrogen with sufficient flow for fluidisation and at this temperature a sample of nitrocarburised AISH13, as prepared above, was immersed in the heated fluidising powder for a period of 4 hours. The sample was cooled in the fluid bed to 350° C. under nitrogen flow and cooled in air. No chromium enrichment of the nitrocarburised surface was experienced as a result of this process.

In a fluid bed reactor of diameter 90 mm and depth 250 mm 380 g of 99.99% purity chromium powder of average particle size 80 microns was mixed with 3.4 kg of 99.99% purity alumina powder of average particle size 125 microns. This fluid bed was heated to 575° C. under high purity nitrogen with sufficient flow for fluidisation and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% flow. This “activation” stage was for a duration of 1 hour. After activation a sample of nitrocarburised AISH13, as prepared above, was immersed in the heated fluidising powder with the hydrogen chloride gas flow being stopped, the heat treatment being at 575° C. for a period of 4 hours. The sample was then cooled in the fluid bed to 350° C. under nitrogen flow and cooled in air. In this trial significant chromium-enrichment (about 70 wt %, refer to quantitative depth profile in FIG. 12) of the nitrocarburised surface was experienced forming a distinct, uniform and continuous 2.5 micron thick layer (FIG. 12). X-ray diffraction analysis indicated the layer was predominately CrN (FIG. 13).

EXAMPLE 4

To assess the potential to increase the process temperature above 575° C. two steel grades were selected having higher tempering resistance than AISI H13 hot work tool steel. Specimens of hardened and tempered (1050° C. autenitised and oil quenched, double tempered at 575° C.) powder metallurgy tool steel Crucible CPM 1V® and conventional ingot metallurgy Bohler-Uddeholm QRO® 90 with a diameter of 38 mm and thickness of 5 mm were nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of each specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of the nitrocarburised samples was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.

In a fluid bed reactor of diameter 90 mm and depth 250 mm 380 g of 99.99% purity chromium powder of average particle size 80 microns was mixed with 3.4 kg of 99.99% purity alumina oxide powder of average particle size 125 microns. This fluid bed was heated to 625° C. under high purity nitrogen with sufficient flow for fluidisation and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% flow. This “activation” stage was for a duration of 1 hour. After activation one nitrocarburised sample of each grade, as prepared above, was immersed in the heated fluidising powder for a period of 4 hours under high purity nitrogen. The samples were cooled in the fluid bed to 350° C. under nitrogen flow and then removed from the fluid bed reactor and cooled in air. In this trial significant chromium-enrichment (about 70 wt %, refer to quantitative depth profile in FIGS. 14 and 15) of the nitrocarburised surface was experienced, with a corresponding nitrogen peak. Compared to processing at 575° C., performing the chromium deposition stage at 625° C. resulted in an increase in layer thickness to approximately 4-6 microns (FIG. 15). Beneath the CrN layer the diffusion zone and core hardness is substantially retained. 

1. A method of forming a diffusion surface layer extending inwardly of an outer surface of a metal substrate, said method including: (i) in an activation stage, providing an activation treatment furnace containing an inert particulate refractory material and a metal based material for forming said diffusion surface layer, said activation treatment furnace having a flow of an inert gas introduced into the inert particulate refractory material and the metal based material in the activation treatment furnace for a first period of time to treat an outer surface region of said metal based material in the presence of a hydrogen halide gas to form an activated metal based material with an activated surface region; and (ii) in a diffusion stage, providing a diffusion treatment furnace and introducing said metal substrate into said diffusion treatment furnace, said metal substrate having been pretreated to form a diffusion zone extending inwardly from the outer surface of the metal substrate in which nitrogen has been diffused to form an inner diffusion zone and an outer compound or white layer formed, at least in part, by an iron nitride, an iron carbide or an iron carbonitride compound without an oxide layer on said outer surface of the metal substrate, treating the metal substrate in the diffusion treatment furnace, sealed against the ingress of atmospheric air and under an inert gas atmosphere, in the absence of hydrogen halide gas for at least a second period of time, in the presence of said activated metal based material, to form said diffusion surface layer on said metal substrate.
 2. A method according to claim 1 wherein the activation treatment furnace is the same as the diffusion treatment furnace.
 3. A method according to claim 1 wherein the diffusion treatment furnace is different to the activation treatment furnace.
 4. A method according to claim 1 further including: (i) in a pretreatment stage, forming said diffusion zone extending inwardly from the surface of the metal substrate in which nitrogen has been diffused to form said inner diffusion zone and said outer compound or white layer of an iron nitride, iron carbide or carbonitride compound; and (ii) treating the metal substrate formed in said pretreatment stage to either prevent formation of a surface oxide on said surface or to remove any said surface oxide formed on said surface prior to said metal diffusion stage.
 5. A method according to claim 1 wherein the inert gas flow in said activation stage is nitrogen and/or argon.
 6. A method according to claim 1 wherein the inert particulate refractory material is aluminium oxide or silicon carbide.
 7. A method according to claim 1 wherein the diffusion treatment furnace contains an inert particulate refractory material that is fluidized by a flow of an inert gas during said metal diffusion stage.
 8. A method according to claim 1 wherein the diffusion treatment furnace contains an inert particulate refractory material that is, at least partly fluidized by vibration means during said metal diffusion stage.
 9. A method according to claim 1 wherein ammonia is not supplied to the diffusion treatment furnace during said metal diffusion stage.
 10. A method according to claim 1 wherein said second period of time is greater than said first period of time.
 11. A method according to claim 1 wherein during said second period of time a hydrogen halide gas flow is pulsed to said diffusion treatment furnace for periods of no hydrogen halide gas flow and at least one period of hydrogen halide gas flow.
 12. A method according to claim 1 wherein said diffusion treatment furnace contains an inert particulate refractory material and an inert gas flow is provided to said diffusion treatment furnace during said second period of time, said inert gas flow being variable between a zero flow rate and a flow rate at or above a minimum fluidization velocity for said diffusion treatment furnace.
 13. A method according to claim 1 wherein a temperature of between 500 and 750° C. is maintained in said activation treatment furnace for said first predetermined period of time.
 14. A method according to claim 1 wherein a temperature of between 500 and 750° C. is maintained in said diffusion treatment furnace for said second predetermined period of time.
 15. A method according to claim 1 wherein said hydrogen halide gas is supplied to said activation treatment furnace continuously during said first predetermined period of time.
 16. A method according to claim 1 wherein said hydrogen halide gas is supplied to said activation treatment furnace during said first predetermined period of time in a pulsed manner for periods of supply separated by periods of non supply.
 17. A method according to claim 1 wherein said hydrogen halide gas is selected from hydrogen chloride gas, hydrogen bromide gas, hydrogen fluoride gas, and hydrogen iodide gas.
 18. A method according to claim 1 wherein the hydrogen halide gas is mixed with a said inert gas prior to entry into the activation treatment furnace.
 19. A method according to claim 1 wherein the hydrogen halide gas is mixed with a said inert gas prior to entry into the diffusion treatment furnace.
 20. A method according to claim 1 wherein ammonium chloride is supplied to said activation treatment furnace during said activation stage, said ammonium chloride being heated while being introduced to disassociate into nitrogen gas and hydrogen chloride gas for activation of said metal based material.
 21. A method according to claim 18, wherein the hydrogen halide gas and the inert gas mixture enters the activation or diffusion treatment furnace at a lower region thereof.
 22. A method according to claim 1 wherein the metal based material for forming the diffusion surface layer is chosen from at least one of: (i) a solid metal or metal alloy; (ii) a metal or metal alloy coated on a substrate carrier; (iii) a particulate or powder metal or metal alloy; (iv) a metal or metal alloy coated on an inert particulate refractory material; (v) a metal halide particle or powder (anhydrous or hydrated); and (vi) a metal halide material (anhydrous or hydrated) coated on an inert refractory particulate material or a substrate carrier.
 23. A method according to claim 1 wherein the metal based material for forming the diffusion surface layer is selected from chromium, titanium, vanadium, niobium, tantalum, tungsten, molybdenum, manganese, and alloys thereof including ferrous based alloys, or metal halides comprised of a metallic element of the aforesaid metals and a halide selected from chlorine, bromine, iodine and fluorine.
 24. A method according to claim 1 wherein the metal substrate is a ferrous based metal or a ferrous based metal alloy.
 25. A method according to claim 1 wherein said inert gas introduced into said diffusion treatment furnace during said second predetermined period of time is nitrogen. 